1. Field of the Invention
This invention relates to methods and apparatus for removing trip hazards in concrete sidewalks and, more particularly, to handheld, flush-cutting concrete saws and dust abatement devices therefor.
2. Description of the Prior Art
Signed into law as Section 12181 of Title 42 of the United States Code on Jul. 26, 1990, the Americans with Disabilities Act (ADA) is a wide-ranging legislation intended to make American society more accessible to people with disabilities. The legislation, which took effect on Jul. 26, 1992, mandates, among other things, standards for access to public facilities, including public sidewalks. The law not only requires that curb cuts be made at intersections and crosswalks to facilitate wheelchair access, but also mandates specifications for slopes and transitions between two surfaces of different levels. Some of the relevant provisions of the law are as follows:                4.5.2 Changes in Level. Changes in level up to ¼ inch (6 mm) may be vertical and without edge treatment. Changes in level between ¼ inch and ½ inch (6 mm and 13 mm) shall be beveled with a slope no greater than 1:2. Changes in level greater than ½ inch (13 mm) shall be accomplished by means of a ramp that complies with 4.7 or 4.8.        4.72 Slope. Slopes of curb ramps shall comply with 4.8.2. Transitions from ramps to walks, gutters, or streets shall be flush and free of abrupt changes. Maximum slopes of adjoining gutters, road surface immediately adjacent to the curb ramp, or accessible route shall not exceed 1:20.        4.8.2 Slope and Rise. The least possible slope shall be used for any ramp. The maximum slope of a ramp in new construction shall be 1:12. The maximum rise for any run shall be 30 inches (760 mm). Curb ramps and ramps to be constructed on existing sites or in existing building or facilities may have slopes and rises as allowed in 4.1.6(3)(a) if space limitations prohibit the use of a 1:12 slope or less.        3-a-1. A slope between 1:10 and 1:12 is allowed for a maximum rise of 6 inches.        3-a-1. A slope between 1:8 and 1:10 is allowed for a maximum rise of 3 inches. A slope steeper than 1:8 is not allowed.        
Public sidewalks and private sidewalks open to the public must comply with the foregoing provisions of the ADA. Tree roots are the single most significant cause of unlevel conditions of sidewalks. Because sidewalks are generally made of contiguous concrete slabs, unevenness typically occurs at the joints between the slabs. Unstable and inadequately compacted soils can also lead to differential settling of adjacent slabs.
Historically, trip hazards caused by uneven lifting and settling of contiguous sidewalk sections have been eliminated either by tearing out the old concrete and replacing it with new slabs having no abrupt transitions between joints, by forming a transition ramp on the lowermost section with macadam, or by creating a chamfer on the edge of the uppermost section. The first method represents the most expensive fix. The second method, which uses dark-colored macadam on a light-colored sidewalk, is unsightly. If the chamfer is made using a surface cutter or grinder, the second method is slow, given that all material removed through grinding must be pulverized. In addition, if the process is performed with a drum cutter, the equipment is relatively expensive and leaves a rough surface. In addition, most equipment used heretofore is incapable of removing the trip hazard over the entire width of a sidewalk. Furthermore, if two adjacent sidewalk slabs have twisted in opposite directions as they have settled or raised, it may be necessary to create a ramp across a portion of the width of the sidewalk on both sides of the joint.
A method and apparatus for removing a trip hazards from concrete sidewalks have been developed by M. Ballard Gardner, and are disclosed in U.S. Pat. application Ser. No. 10/155,663, which is identified above. Using the method and apparatus, a trip hazard may be removed over the entire width of a sidewalk, and portions of two concrete slabs intersecting at a common joint may be chamfered, without necessitating the pulverization all material removed during the chamfer operation. A right-angle grinder motor, in combination with a specially-designed hub and a circular diamond-grit-edged blade, is employed to chamfer the trip hazard in a flush-cutting operation.
Referring now to FIG. 1, a typical right-angle grinder motor 100 is shown. The grinder motor 100 has a body 101, which encloses an electric drive motor having a generally horizontal output shaft and a cooling fan (neither of which are visible in this view). A right-angle gear train assembly 102 is attached to the front of the body 101. The right-angle gear train assembly 102 is coupled to the horizontal output shaft, and has a generally vertical, rotatably-powered, threaded output spindle 103. The grinder motor 100 also has a handle 104, a power switch 105, motor brush caps 106, cooling vents 107, and an electrical power cord 108. Although the method and apparatus for removing trip hazards is described in connection with an electrically-powered right-angle grinder motor, a compressed-air-powered right-angle grinder motor may be employed with equally satisfactory results.
Referring now to FIGS. 2 through 6, a unique hub 200 is designed for installation on the threaded output spindle 103 of an angle grinder, such as the electric grinder motor 100 shown in FIG. 1. The hub 200 has an attachment collar 201 that is unitary and concentric with both a blade mounting flange 202 and a blade centering shoulder 203 on the flange 202. A central mounting aperture 204 passes through the collar 201, the flange 202, and the shoulder 203. The mounting aperture 204 is threaded to receive and engage the threaded output spindle 103 of the right-angle grinder motor 100. The attachment collar 201 has at least one pair of flattened parallel sides 205 for receiving a wrench used to tighten the hub 200 on the output spindle 103. The side 206 of the blade mounting flange 202 opposite the collar 201 is equipped with at least two, and preferably three to six, countersunk holes 207, by means of which a generally circular, diamond-grit-edged rotary blade may be attached with countersinking screws and self-locking nuts (not shown in this drawing figure).
Referring now to FIG. 7, a rotary blade 700 is equipped with a central positioning aperture 701 sized to fit over the blade centering shoulder 203 with a generally minimum amount of clearance required for a non-interference fit. The blade is equipped with non-threaded countersunk holes 702 which align with the threaded countersunk holes 202 on the blade mounting flange 202. Countersinking screws (shown in FIG. 8) are employed to affix the blade 700 to the blade mounting flange 202. When fully tightened in the countersunk threaded holes 202 in the flange 202, the heads of each of the screws is flush with the surface of the blade 700. Although it is possible to countersink only the holes 702 of the saw blade 700 and use specially designed screws having a very shallow countersinking head, conventional countersinking screws have greater structural integrity. The edge 703 of blade 700 is formed from a metal matrix which incorporates diamond grit throughout, which enables the blade, when rotating, to cut through “green” or seasoned concrete. For a presently preferred embodiment of the blade, the new diameter is 8 inches (about 203 mm), and the blade core has a thickness of about 0.55 inch. The height of the blade centering shoulder 203 is preferably also about 0.055 inch. If the blade centering shoulder were to protrude through the blade, the edges thereof would become peened over the edges of the blade centering aperture 701, thereby making removal of the blade difficult.
Referring now to the exploded assembly 800 of FIG. 8, an electrically-powered right-angle grinder motor 100 is shown together with the hub 200, the blade 700, multiple countersinking blade-attachment screws 801 and multiple self-locking nuts 802, all positioned for assembly as a unit. It will be noted that each of the self-locking nuts has a deformable polymeric insert 1005, which provides the self-locking function.
Referring now to the assembled concrete saw 900 of FIG. 9, the hub 200 has been installed on the output spindle 103 of the right-angled grinder motor 100, and the blade 700 has been secured to the hub 200 with the countersinking screws 801 and the self-locking nuts 802. It will be noted that the lower surface 901 of the blade 700 is completely flat, with no attachment hardware protruding below its surface. By definition, the lower surface 901 of the blade 700 is “flush-mounted” on the hub 200.
Referring now to FIG. 10, the portion of FIG. 9 within the ellipse 10 is shown in cross-sectional format. In this detailed view, it is clearly seen that the attachment collar 201 is unitary and concentric with the blade mounting flange 202 and the blade centering shoulder 203 on the flange 202. The threads 1001 within the central mounting aperture 204, which have spirally engaged the threads 1002 on the output spindle 103, are clearly visible in this view. It will be noted that the head 1003 of each countersinking blade attachment screw 801 has a socket 1004. The blade attachment screws 801 are inserted through the countersunk holes 702 in the blade 700, through the holes 207 in the blade mounting flange 202 and secured with the self-locking nuts 802. Using an allen-type wrench which engages the sockets 1004, the screws 801 may be kept from rotating while the self-locking nuts 802 are tightened against the upper surface of the blade mounting flange 202, thereby securing the blade 700 to the hub 200. It will also be noted that the central positioning aperture 701 in the blade 700 is sized to fit over the blade centering shoulder 203 with a generally minimum amount of clearance required for a non-interference fit.
Referring now to FIG. 11, it will be noted that, at the junction of a first concrete slab 1101 and a second concrete slab 1102, there is a trip hazard 1103 that has been caused by the first slab 1101 being raised with respect to the second slab 1102. Removal of the trip hazard, by making a dry chamfer cut on the first concrete slab 1101, will now be described in detail with reference to the remaining drawing figures. The chamfer, when complete, will have a 1:8 rise. Both slabs 1101 and 1102 rest on a substrate 1104 of gravel, sand or soil. Using the concrete saw (i.e., the right-angle grinder motor 100 with the hub 200 and blade 700 mounted thereon), a first chamfer cut 1105 is made on the edge of concrete slab 1101, which has raised with respect to the second concrete slab 1102. It should be understood that cuts with the concrete saw 900 are made from right to left, as the operator kneels on the high side of the sidewalk. It will be noted that the bottom surface of the blade 901 is in close proximity to the lower cut surface 1106. However, as heads 1003 of the blade-attachment screws 801 are flush with the lower surface of the blade 700, they are shielded from abrasive action of the concrete within the cut 1105. In order to protect the hub 200 from abrasion by the concrete, the cut must stop before the rotating hub 200 contacts the upper edge 1107 of the cut concrete. Using a blade having a diameter of about 8 inches (about 203 mm), a 2.375 inch deep cut may be made without endangering the hub.
Referring now to FIG. 12, the blade has been removed from the cut 1105. It will be noted that a first cantilevered ledge 1201 extends over the cut 1105.
Referring now to FIG. 13, the cantilevered ledge 1201 has been fractured by hitting it with a hammer or other similar instrument.
Referring now to FIG. 14, a second chamfer cut 1401 is made, which is a continuation of the first chamfer cut 1105. Once again, in order to protect the hub 200 from abrasion by the concrete, the cut must stop before the rotating hub 200 contacts the upper edge 1402 of the cut concrete.
Referring now to FIG. 15, the blade has been removed from the cut 1401. It will be noted that a second cantilevered ledge 1501 extends over the cut 1401.
Referring now to FIG. 16, the second cantilevered ledge 1501 has been fractured by hitting it with a hammer or other similar instrument.
Referring now to FIG. 17, a third chamber cut has been made which removes the remainder 1701 of the trip hazard 1103.
Referring now to FIG. 18, the first concrete slab 1101 is shown with the a completed chamfer cut 1801. The cutting equipment, which consists of the right-angle grinder motor 100, the attached hub 200 and blade 700, have been removed, as have been the trip hazard debris pieces 1201, 1401 and 1701.
With training, a skilled worker can make an angled chamfer cut into the edge of a raised concrete slab, so that a smooth transition between a lower slab and the raised slab may be formed. Trip hazards of slightly more than 2.54 cm height can be removed in using three cuts with an eight-inch blade. Trip hazards of nearly two inches in height can be removed with additional cuts, using the invention as heretofore described.
As the trip hazard removal method involves cutting the concrete with a rotating diamond-edged circular saw blade, a considerable amount of dust is created. Because concrete is a mixture of hydrated (i.e., crystalized) cement, aggregate (gravel) and silica sand, the dust contains both cement dust and silica dust. As statistical evidence has shown that the breathing of silica dust can cause lung cancer, it is essential that the saw operator and those in the vicinity of the work be protected from the dust. Although it is fairly simple to provide the saw operator with eye protection and a dust mask, it is more difficult to ensure that all who are near the work area receive protection. Furthermore, as masks are typically not 100 percent effective, dust abatement is a better solution.